Why is the planet green?
What if I told you that the answer to this question, originates from an experiment conducted using
starfish?
In 1963, Professor Robert Paine stood on the shore of the Makah Bay in Washington. In the intertidal zone he observed a diverse community of species including purple and orange sea stars, mussels, barnacles, limpets, anemones and algae. Paine noted that the sea stars (Pisaster orchraceus) were at the top of the food chain in this community, and that mussels were the most important component of their diet.
During the experiment, Paine would stand on the shore of Makah Bay, and throw the sea stars back into the ocean. As time passed and the P. orchraceus population in the intertidal zone decreased, Paine observed biodiversity in the community diminish until the intertidal zone was dominated by a monoculture of mussels; the sea star’s prey.
What Paine had discovered was that the presence of the P. orchraceus - the apex predator - controlled the composition of a community. In other words, Paine discovered that not all species are equal. He identified these species as “keystone species.” Keystone species are apex predators which impact the presence and abundance of other species in an ecosystem, through “trophic cascades.”
Trophic Cascades
A trophic cascade refers to the cascading impact that an apex predator has on the trophic levels in a food chain. An apex predator has a direct negative impact on the prey which it consumes. The impact of this feeding event cascades down the food chain. The apex predator also has an indirect and positive impacts the prey’s prey. Impact refers to species presence and abundance; a positive impact indicates an increase in population, whereas a negative impact indicates a decrease in population.
Green World Hypothesis
In returning to our question – why is the planet green – it is because apex predators have a top-down effect on the primary producer population. This is also known as the Green World Hypothesis.
Let’s simplify the trophic levels of green ecosystem such as a grassland ecosystem; the plants are the primary producers, grasshoppers would be considered herbivores and rats which would be the predators. The rats eat the grasshoppers, and the grasshoppers eat the plants. However, when the rat eats the grasshopper, this also prevents the grass from being eaten by the grasshopper. In other words, the predator regulates the herbivore population, which in turn regulates the primary producer population. Put even simpler, the rat population controls how many plants are observed in the grassland
ecosystem.
What We Have Learned
The planet we know and observe today is made up of many complex ecosystem interactions. From the food chains that exists in our ecosystems, to the consequences of throwing starfish into the ocean, we must recognize that every action has an equal and opposite reaction. As stewards of this planet, we must seek out the downstream effects of our actions. Because when it comes to nature, there is more to what we observe than meets the eye.